1. Field of the Invention
This invention relates to a control system for a continuously variable transmission of the type having two conical variable diameter pulleys driveably connected by a belt for producing variable speed ratios as the relative diameters of the pulleys change. In particular, this invention relates to such a transmission control system that includes an electronic control unit, and more particularly pertains to such transmission control systems that include hydraulic control valves for actuating the various components of the transmission that establish the operating speed ratio of the transmission.
2. Description of the Prior Art
A control valve system for a continuously variable transmission of this type is described in German Offenlegungsschrift No. 3310786. In that system, the primary shaft is driven directly by an internal combustion engine through a torque sensor. The transmission includes variable displacement cylinders associated with each of two pairs of cone pulleys, which are alternately pressurized and vented through the operation of control valves supplied with pressurized hydraulic fluid from the hydraulic pump. Control signals, determined by a torque requirement and by the road speed of the vehicle, are generated in electronic form by an electronic control unit whose output determines the magnitude of the electrical voltage or current supplied to the windings of a solenoid-operated main control valve.
This system has no provision that would permit any emergency operation in the event of failure of the electronic control unit.